Engine issues Diagnostic

Reply Subscribe

Thread Tools

Search this Thread

Mar 1, 2026 | 04:25 PM

78Gold

Thread Starter

6th Gear

Joined: Sep 2025

Posts: 10

Likes: 16

From: Sydney

Engine issues Diagnostic

Hi All,
I am running aRivera Eliminator II SU Carburettor. (1997 and later version). Upswept manifold now fitted. Originally bought to install on a 2003 1200s
Now fitted to a 1989 FXRP with 1340 Evolution Motor. Have gone one size up on needle (By others)
I need help diagnosing issues.
On accelleration at about 2/3 throttle I get missing and like a backfire. Sounds like detonation at wrong timing.
For example it gets worse at 85kph in top gear and then gets better again at 105kph in top.
It got worse after I had a voes hose come of. It was noticeable but not as bad with an older style manifold ( with rubbers) but seems worse going to the upswept manifold.
This upswept manifold should have been an improvement but wasnt.
I noticed that the Voes is stretched tight and I am not sure if this is causing vacuum issues.

I am thinking of the following diagnostic trail, have I missed anything ?
1. Test Voes switch and routing of Voes hose.
2. Bypass manifold port and run Voes direct to Carby (If port)
3. Check hose not "Collapsing"
4. Using "Start ya Bastard" or similar see if any leaks around manifold

Basically I am thinking
A. VOES not switching correctly due to weak or unstable vacuum signal
OR
B. small intake leak introduced during manifold change

Anyone else running and Eliminator ii or experience with similar issues on the 1340 help with anything I may be missing.

Thanks

Mar 2, 2026 | 10:04 AM

98hotrodfatboy

Seasoned HDF Member

Joined: Mar 2010

Posts: 21,095

Likes: 7,622

From: Poolville

I'm not familiar with the jetting circuits on an SU and don't know anybody that's running one. What was wrong with the original CV carb?? Or replace it with an S$S.....

Mar 2, 2026 | 10:04 PM

78Gold

Thread Starter

6th Gear

Joined: Sep 2025

Posts: 10

Likes: 16

From: Sydney

Hi,
Nostalgia and Aesthetics.
As a much younger man I raced MG's and always ran SU carbs and had a few other cars over the years running SU's, Strombergs etc.
I never had any issues like this with them.
They were Oil filled though and the Eliminator II does not have any oil as such relying on a spring, it sounds mechanically "sloppy" but was working well until recently.
I have re-jetted as originally was on a 1200s but that didn't solve the issue.
My idea is it's got to be Voes, Manifold leak or Timing somehow but have not been able to sort.
This is on a swing arm chop so nothing is built to be practical as such.

Mar 2, 2026 | 11:54 PM

Aussiemadness

Cruiser

Joined: Dec 2014

Posts: 159

Likes: 95

From: Sydney, Australia

I would go through the system and make sure timing/voes and all vacume lines and manifold seals are ok, also carb mounting to manifold.
No need to re-jet as Sportster and BT use the same main jet (.100) and spring, red I think from memory, if you have a bigger cam and/or head work I would put in the silver spring as this will richen her up a bit, keep the .100 main jet. I used the silver spring and 100 main on my 91 evo with EV27 and she ran great.

You may need to reset your main jet stock setting and then re-adjust, just screw the main in (clockwise) until she lightly hits bottom and then crew out 6 full turns, start her up and adjust until she's ticking over nice and take her for a run and do a plug read, either richen her up (clockwise) or lean her (anti-clockwise).

Hope this helps.

Edit: spray some WD40 inside the dome, this will help with the carb piston slap.

Last edited by Aussiemadness; Mar 2, 2026 at 11:56 PM.